In recent years, environmental issues have become a great concern on a global scale. In the automotive industry, attempts for environmental protection in manufacturing processes have been actively promoted. In automotive manufacturing processes, problems such as global warming, industrial waste, and discharge of volatile organic compounds (VOC) have arisen. In particular, reduction of the amount of VOCs, most of which are mainly released during coating processes, has become an urgent task.
To impart corrosion resistance and a beautiful appearance, the outer panels of automobile bodies are usually coated with a multilayer coating film comprising an undercoat of a cationic electrodeposition coating composition, an intermediate coat, and a topcoat. In view of VOC reduction, use of water-based coating compositions is also promoted for intermediate and top coating compositions.
However, compared to organic solvent-based coating compositions, conventional aqueous coating compositions are unsatisfactory in finished appearance of the resulting coating films, due to water being used as the main solvent.
As an aqueous coating composition that has good coating workability and that can form a coating film with an excellent finished appearance, such as with excellent smoothness, Patent Literature 1 discloses a water-based coating composition consisting essentially of a specific acrylic and/or polyester resin, a specific polycarbonate resin, and a curing agent. However, such a water-based coating composition may provide an unsatisfactory finished appearance. Patent Literature 2 discloses an aqueous coating composition consisting essentially of a specific acrylic and/or polyester resin, a specific polycarbonate resin, a curing agent, and specific resin particles. However, the coating film formed using this coating composition may have a poor finished appearance, such as with a low degree of smoothness.
A method for forming a multilayer coating film by a 3-coat-2-bake (3C2B) method is widely used as a method for forming a coating film on automobile bodies. This method comprises the following steps after applying an electrodeposition coating to a substrate: application of an intermediate coating composition→curing by baking→application of an aqueous base coating composition→preheating (preliminary heating)→application of a clear coating composition→curing by baking. However, in recent years, for the purpose of energy savings and VOC reduction, attempts have been made to use an aqueous intermediate coating composition as an intermediate paint and omit the bake-curing step that may be performed after application of the intermediate coating composition, thereby using a 3-coat-1-bake (3C1B) method comprising the following steps after applying an electrodeposition coating to a substrate: application of an aqueous intermediate coating composition→preheating (preliminary heating)→application of an aqueous base coating composition→preheating (preliminary heating)→application of a clear coating composition→curing by baking (see, for example, Patent Literature 3).
In a 3-coat 1-bake method using an aqueous intermediate coating composition and an aqueous base coating composition as described above, an aqueous intermediate coating composition comprising a specific water-dispersible polyurethane composition has been proposed as an aqueous intermediate coating composition that has excellent removability in cleaning of coating guns, etc., and that can form a multilayer coating film with excellent chipping resistance and excellent coating film appearance; a method for forming a multilayer coating film using this aqueous intermediate coating composition has been also proposed (see, for example, Patent Literature 4).
However, the above method for forming a multilayer coating film using the aqueous intermediate coating composition may have a problem. That is, permeation of the solvent contained in a clear coating composition causes swelling of intermediate and base coating films, thus forming minute surface roughness, and thereby reducing the smoothness of the resulting multilayer coating film, which results in an unsatisfactory finished appearance and/or poor removability of the aqueous intermediate coating composition in bell cleaning.